Whilst a drill string is being inserted into the ground, or even once this string of drilling rods or a working pipe casing has been installed, various measurements on the drill string or pipe casing may be carried out. For example, measurements can be made to determine whether a rod is stuck by virtue of a cave-in at depth, perhaps several thousand meters from the point of origin of the drilling.
It may also be useful to detect the position of the joints of the drill string or of the pipe casing. In fact, a string of drilling rods or a working pipe casing may comprise a collection of rods or tubes—having, for example, lengths of the order of around ten meters, which are screwed onto one another, and the counting of the joints constitutes a fixing of position.
It may also be useful to fix the locations of perforations or of zones of weakening, via corrosion of the pipe casings. It may also be useful to know the stress condition at a point in a drill string whilst it is being raised from the point of origin, for example, in order to create a neutral point at a given depth.